[Leetcode 144]二叉树前序遍历Binary Tree Preorder Traversal

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        LinkedList<Integer> ans=new LinkedList<Integer>();
        Stack<TreeNode> tmp=new Stack<TreeNode>();
        while(root!=null){
            ans.add(root.val);
            if(root.right!=null){
                tmp.push(root.right);
            }
            root=root.left;
            if(root==null&&!tmp.isEmpty()){
                root=tmp.pop();
            }
        }
        return ans;
    }
}